The quality of a structure, whether it is a house, apartment building or commercial office building, is inextricably tied to its foundation. If the structure is not built on a proper foundation, the rest of the structure, even if properly constructed, is likely to show defects over time. When foundations are constructed directly on soils or on the ground, it creates an unstable environment for the foundation. In addition, if these soils are active or expansive, the environment may be especially problematic. For example, in regions where the soil has a high percentage of active clay, expansion and contraction of the clay subjects the foundations to significant loads (forces) and potential movement.
Structures built on soils in certain regions may have had their slab foundations and walls displaced and damaged (e.g. foundations and walls cracked) as a result of differential expansion and/or contraction of the soil. This damage, not surprisingly, creates consternation for the home or building owner and, in turn, other parties such as the developer, the builder, the salesperson, and insurer of the structure. Often, litigation ensues amongst these parties. Over time, engineers developed systems and methods for designing foundations in an attempt to minimize damage due to movement of soils. Some of these system and methods include very heavy slabs which include extensive steel reinforcement and large concrete beams in the slab; injection of water to cause pre-installation expansion of soils, injection of stabilizing chemicals to cause pre-installation expansion and stabilization of soils; installation of piers (piles) to stabilize the slab and prevent the slab from dropping should the soils contract; isolation of the slab from the active soils by suspending the slab above the ground with the use of piers and void space under the slab. In addition, systems and methods have been developed to repair and isolate the slab foundations from the active soils after such movement of the soil has occurred.
The installation of piers to suspend or lift the slab foundation to create a protective void between the soil and the slab foundation is a solution that is commonly applied. This method may also mitigate some slab foundation failures caused by seismic activity such as earthquakes and tremors, as such seismic activity, too, causes the soil to move in a manner that can damage a slab foundation. U.S. Pat. No. 7,823,341, HEIGHT-ADJUSTABLE, STRUCTURALLY SUSPENDED SLABS FOR A STRUCTURAL FOUNDATION, issued Nov. 2, 2010 ('341 Patent), which is incorporated by reference herein, discloses a method of lifting a foundation by piers.
There are several types of pier or piles used in foundation construction including poured concrete piers, metal helical piers, metal pressed or pile driven piers. The most common pier in residential and commercial building construction is the poured concrete pier. The typical diameter of a concrete pier is 12″ to 24″, however 36″ diameter piers or larger can be specified in very heavy designs such as multistory garages. When designing a foundation on piers, two load factors for the piers must be considered: axial load and lateral load. Axial load is caused by the live and dead loads of the structure and the contents of the structure which put vertical load on the piers while lateral load is caused by horizontal loads on the structure that transfers to the piers such as wind loads on the structure. A 12 inches to 24 inches diameter concrete pier typically has sufficient amount of lateral strength to withstand the lateral loads that can be transferred from the structure. However, metal helical or pressed piers have a much narrower out-to-out dimension (typically <6 inches) and therefore may not have sufficient lateral strength to withstand the lateral loads. In these cases, a method may be implemented to increase the lateral capacity of a metal pier.
FIG. 1 shows a prior art system for suspending slab foundations as described in the '341 Patent. As the '341 Patent describes, prior to forming slab foundation 16, piers 10 are installed into ground 15. Metal helical piers 10 include shafts 11 attached to helixes 12. Piers 10 may be installed by applying a torque to shafts 11, which causes helixes 12 to drill into ground 15 to a desired depth, usually a depth to provide proper load bearing capacity. Lifting mechanisms 13 are installed on top of piers 10. Concrete is then poured within form boards that are placed to define the perimeter of slab foundation 16. Once the concrete is poured to cast slab foundation 16, it is allowed to cure. As the concrete cures and strengthens, it fixes to lifting mechanism 13, which, as noted above, sits on top of pier 10.
After slab foundation 16 is formed on ground 15, lifting mechanisms 13 are used to lift slab foundation 16 to a desired height above ground 15, thereby creating void 14 between ground 15 and slab foundation 16. Lifting mechanisms 13 include lifting bolts 17. To lift slab foundation 16, torque is applied to lifting bolts 17, screwing them into and through nuts 18 until lifting bolts 17 make contact with lifting plates 19, which rests on top of piers 10. On application of sufficient torque, lifting bolts 17 turn on lifting plate 19 until it lifts slab 16 up from ground 15. As lifting mechanism 13 lifts slab foundation 16, void 14 is created. After the lifting of slab foundation 16, ground 15 is able to expand into void 14. Thus, with void 14 in place, expansion of ground 15 does not cause an upward load on slab foundation 16.
If narrow metal piers are used and it is determined that they will not have sufficient lateral capacity to withstand lateral loads, the metal piers may be pre-stabilized before the slab is constructed. A common method to stabilize a metal pier is to pour concrete around the top of the helical to form a concrete cap to the metal pier. This concrete cap is designed to increase the lateral capacity of the metal pier by keeping it from buckling, moving or bending under lateral loads. When the soil is active, however, placing stabilizing devices in it may cause more problems than it solves. Specifically, expansion of the active soil may push the stabilizing device upwards (e.g. a concrete cap), causing the pier and the load it is supporting (e.g. house) to be displaced upwards.
As noted above, a separate method of preventing damage to slab foundations caused by movement of active soil is to treat the soils with either water or chemicals in an attempt to stabilize the soils. Moisture conditioning or water injection is used to wet the soil thereby cause pre-swelling of the soil. Once the soil has been treated with water and it has expanded, plastic is placed over the soil in an attempt to maintain the moisture condition. Chemical injection of the soils is used to modify the properties of the soil. For example, these chemicals change the ability of clay to expand or contract depending on the amount of water present in the clay. The chemicals adjust the clay's inherent capacity to expand as its water content increases and to contract as its water content decreases. An example of a chemical composition used to modify the properties of soil and in order to stabilize the soil is disclosed in US Patent Pub. No. Application No. 2012/0288335 ('335 Application), filed May 11, 2012 by Rodney Green. The systems and methods of stabilizing soil by compositions added to soil disclosed by the '335 Application is incorporated herein by reference. However, it should be noted that compositions other than those disclosed in the '335 Application may be used in embodiments of the invention described herein.
The composition disclosed in the '335 Application, when added to soil, creates an exchange of ions that changes the molecular structure of the soil. This change in structure prevents the soil from absorbing water. Thus, the movement of soil caused by its contraction and expansion due to changes in water content is eliminated.